1. Technical Field
The present invention relates to a receiver for regenerating a signal wave from a received wave, and in particular to a receiver for regenerating a signal wave via digital signal processing.
2. Related Art
Conventionally, an AM receiver of the superheterodyne system shown in FIG. 3 is known as a receiver for receiving a broadcast wave transmitted from a broadcast station and regenerating a signal wave in the audio frequency band.
The AM receiver regenerates a signal wave SAF via analog signal processing. The receiver supplies the AM broadcast wave SAF received by an antenna 1 to a frequency converter 3 via a high-frequency amplifier 2 and mixes the signal with a channel selection signal So of the local oscillation frequency from a local oscillator 4 in order to frequency-convert the signal to a signal of the intermediate frequency (455 kHz), hereinafter referred to as an IF signal. The AM receiver removes unwanted frequency components by passing the frequency-converted IF signal SIF through a band-pass filter 5 in the intermediate frequency band, amplifies the resulting signal via a gain control intermediate frequency amplifier 6, removes external noises via a noise clamping circuit 7, then detects the signal via a detection circuit 8 to regenerate a signal wave SAF in the audio frequency band. The AM receiver voltage-splits the signal wave SAF via fixed resistors 10, 11 of predetermined values and outputs the resulting signal.
Further, the AM receiver converts the signal wave SAF detected by the detection circuit 8 converts to a DC voltage VAGC via a low pass filter 9 to obtain a constant (non-variable) signal wave SAF.
Assuming the amplification ratio of the gain control intermediate frequency amplifier 6 is fixed, a variation in the amplitude of the AM broadcast wave SRF in accordance with a variation in the receiving intensity causes a variation in the signal wave SAF even in case the AM broadcast wave SRF of the same detection data level is detected.
In order to prevent such a disadvantage, a constant (non-variable) signal wave SAF is obtained by lowering the amplification ratio of the gain control intermediate frequency amplifier 6 when a DC voltage VAGC is raised, and by raising the amplification ratio of the gain control intermediate frequency amplifier 6 when the DC voltage VAGC is lowered.
[Problems that the Invention is to Solve]
The inventor of the application has replaced a received composed of the aforementioned analog circuits with one composed of digital circuits to make an attempt to directly regenerate the digital signal wave SAF that is compatible with for example digital audio apparatus.
More precisely, the inventor has digitized a receiver itself, without providing an additional feature whereby an analog signal wave SAF output from the detection circuit 8 in FIG. 3 is converted to a digital signal wave via an A/D converter, thus directly outputting a digital signal wave SAF. The inventor, in doing so, has attempted to develop a more high-accuracy receiver that was not available via an analog receiver.
The inventor, as one of such attempts, has proposed to implement high-accuracy signal processing obtained through digital arithmetic operation, by replacing the gain control intermediate frequency amplifier 6, the noise clamping circuit 7 and the low pass filter 9 with digital circuits.
The proposal presented a problem. While frequency of each signal supplied to the gain control intermediate frequency amplifier 6, the noise clamping circuit 7 and the low pass filter 9 is down-converted to an intermediate frequency (455 kHz) via the frequency conversion in the frequency converter 3, the frequency is too high for general digital circuits to process. This requires introduction of extremely high-speed digital circuits, not just replacement of existing circuits.
In order to perform the digital signal processing equivalent to that in the gain control intermediate frequency amplifier 6, the noise clamping circuit 7 and the low pass filter 9 as analog circuits according to the related art, it is necessary to digitize the IF signal SIF output from the band-pass filter at a sampling frequency at least double the intermediate frequency (455 kHz), for example a sampling frequency of 1 MHz according to the Nyquist sampling theorem.
Thus, there was a problem that high-speed and expensive digital circuits were required, which made it difficult to digitize a receiver.
For example, an approach for high-speed digital signal processing provides a plurality of digital circuits that can perform high-speed arithmetic operation in parallel and implements substantial high-speed arithmetic operation through parallel processing using these plurality of digital circuits. This approach requires complicated control for obtaining precise synchronization between digital circuits and invites high costs due to expanded circuit scale.
The invention aims at solving the problems involved in the related art and providing a new receiver of a simple configuration that can directly output high-accuracy digital signal waves.